This invention relates to a process for improving the effectiveness of a catalyst composition. More specifically this invention relates to a process for improving the productivity of a molybdenum-based catalyst by an improved regeneration operation.
The method used to regenerate catalysts depends to a great extend on the reaction for which it is a catalyst since it is the actual use in the reaction which causes de-activation. Thus regeneration methods of use with catalysts used in an alkylation reaction for example would not be expected to be effective in regenerating an ammoxidation catalyst.
Most catalysts used in ammoxidation processes are found not to maintain a uniform level of activity in use but to drop off in effectiveness after a number of days use. The catalyst is at that stage designated at "spent". It has been found possible to restore a substantial proportion of the activity of many such spent catalysts by a simple regeneration process. Most commonly this involves simply heating the spent catalyst in air at an elevated temperature, for example for 300.degree. to 900.degree. C or perhaps even higher. The temperature and time of heating are generally selected to minimize any side effects on the inert substrate upon which such catalysts are conventionally supported.
One regeneration method for such catalysts is disclosed in Def. Pub. No. T 784,946. In the document the method described involves heating the deactivated catalyst at a high temperature (350.degree.-650.degree. C is declared appropriate) with a reducing gas or gaseous mixture.
The more common approach, however, is to use a non-reducing atmosphere, such as dry air, as is used in the regeneration apparatus described in U.S. Pat. No. 3,141,742.
In U.S. Pat. No. 3,882,159 a method of regenerating molybdena-containing catalysts is described which comprises heating spent catalyst in a fluidized bed in the presence of fluidized particles comprising molybdena and an inert support. The heating is conducted in air and steam which is said to facilitate the transfer of molybdena to the spent catalyst particles from the fluidized particles.
The present invention provides a process for regenerating the activity of a spent molybdenum-containing ammoxidation catalyst which does not depend on the addition of a further catalyst component.
The process of the invention has the advantage that the yield of the desired product based on the ammonia and olefin starting materials, is significantly increased. In an area where a yield improvement of a single percentage point can mean an increase in profitability of a plant in the order of several hundred thousand dollars over a full year, the enhanced performance of the catalysts that have been subjected to the process of the invention is regarded as extremely significant. A further advantage is that the catalysts so regenerated appear to have increased stability and a decreased tendency to burn ammonia when used in an ammoxidation reaction.
The process of the present invention results in a catalyst with a slightly decreased activity (as reflected by conversion) but a greater selectivity (such that the efficiency of the overall ammoxidation process is increased) by comparison with conventionally regenerated catalysts, especially when used in ammoxidation reactions operated under superatmospheric pressures.